The most distinctive property of a superconductive material is the near total loss of electrical resistance by the material when at or below a critical temperature. This critical temperature is characteristic of the material and is referred to as the superconducting transition temperature of the material, T.sub.c.
The history of research into the superconductivity of specific materials began with the discovery in 1911 that mercury superconducts at a transition temperature of about 4.degree. K. In the 1920's NbC was found to superconduct at a higher temperature, namely up to about 10.5.degree. K. Since that time, many applications for the phenomena of superconductivity have been conceived which could not be commercialized because of the extreme low transition temperatures of the superconductive material.
Although many materials have been examined in an effort to find compounds which will superconduct at higher, more practical temperatures, the highest temperature superconductor known until about 1986 was Nb.sub.3 Ge having a critical temperature, Tc, of approximately 23.3.degree. K. Superconducting devices utilizing Nb.sub.3 Ge as the superconductor, like those devices which employed the superconductors preceding Nb.sub.3 Ge, required the use of liquid helium as refrigerant-coolant in commercial applications. In 1986 Bednorz and Muller disclosed that certain mixed phase compositions of La-Ba-Cu-O appeared to exhibit superconductivity at about 30.degree. K. Investigation of that system established that the crystalline phase therein responsible for superconductivity had a crystal structure like that of K.sub.2 NiF.sub.4 (214). The upper temperature limit of onset, T.sub.co, for superconductors of a 214 type crystalline structure has been found to be about 48.degree. K.
Following the discovery of superconductivity in such rare earth-alkaline earth Cu oxide systems of a 214 crystalline structure, a new class of rare earth-alkaline earth-copper oxides was discovered which were superconductive at temperatures above 77.degree. K. This new class of rare earth-alkaline earth-copper oxides, commonly referred to as "123" high-temperature superconductors, have perovskite related crystalline structures. The unit cell consists of three sub-cells stacked one above the other along the C-axis. X-ray crystallographic and neutron powder diffraction studies indicate the structure to be oxygen deficient and that the ordering of oxygen in the basal planes is critical to the oxide exhibiting superconductivity properties. See C. Poole et al, Copper Oxide Superconductors (John Wiley & Sons 1988). The unit cell formula of the 1,2,3 compound is
L.sub.1 M.sub.2 Cu.sub.3 O.sub.6 +.delta. (.delta.=0.1 to 1.0, preferably about 1.0)
wherein L is scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium, and M is barium, strontium or mixtures thereof. The studies indicate that when .delta. is between about 0.1 to about 0.5, the resulting 1,2,3 compound exhibits a tetragonal unit cell crystallographic symmetry and is non-superconductive. In this unit cell, the lattice dimensions of the C-axis is approximately 11.94 angstroms and the a and b axes are approximately 3.9 angstroms. When .delta. is between 0.5 and 1.0, the resulting 1,2,3 compound has an orthorhombic unit cell crystallographic symmetry and is superconductive. The orientation of the oxygen atoms in the unit cell causes the cell to compress slightly along the a axis and thus the lattice dimension of the a axis is less than that of the b axis. Lattice constants a=3.80, b=3.86 and c=11.55 have been reported for the orthorhombic unit cell.
With the discovery of the 123 class of high temperature superconductivity compositions it has become possible to economically pursue many previously conceived applications of the superconductivity phenomena which were commercially impractical wherein cooling by liquid helium was required. Since they superconduct at temperatures greater than 77.degree. K, the new "123" class of high temperature superconductors may in practical applications be cooled with liquid nitrogen--a more economically feasible refrigerant. As a result, the rather complex thermal insulation and helium-recycling systems employed with conventional superconductors, in order to avoid wasting the expensive helium coolant, has been abandoned, thereby greatly simplifying and enhancing the reliability of commercial superconductors.
However, the heretofore high temperature superconductors have been impractical in some applications due to their inability to (1) carry high current loads in intense magnetic fields, (2) entrap strong magnetic fields, and (3) exhibit low high-frequency surface resistance. As a result, significant commercial barriers against use of the 123 superconductors in numerous applications, such as in magnets, magnetic separators, transmission lines, high frequency generators and magnetically levitating trains (meglav) exist.
In magnetic separators, for example, superconductors are required to have a current density, J, between about 33,000 and 66,000 amps/cm.sup.2 in a magnetic field between 2 and 3 T. In order to be commercially practical in transmission lines and high frequency generators, the superconductive material must further exhibit a very small high frequency surface resistance at or below its critical temperature. To be practical in magnets, superconductive materials must, in addition to being lightweight, be capable of entrapping within their crystalline structure a high magnetic field. [While superconductive materials of low T.sub.c are reported as being capable of entrapping fields as high as 22,400 Gauss (see, for example, M. Rabinowitz, E. L. Garwin and D. J. Frankel, Lettere Al Nuovo Cimento, 7, 1, (1973); E. L. Garwin, M. Rabinowitz, and D. J. Frankel, Appl. Phys. Lett., 22, 599 (1973); M. Rabinowitz, H. W. Arrowsmith and S. D. Dahlgren, Appl. Phys. Lett., 30, 607 (1977); and M. Rabinowitz, IEEE Magnetics, 11, 548 (1975)), such superconductors are not commercially viable for use in magnets due to their low T.sub.c as well as their inability to carry high current loads.]